datum

Datum, in surveying, refers to a reference point or surface used to establish elevations and positions. For vertical control, a datum is a level surface, often mean sea level, from which heights are measured. This is crucial for large-scale surveys where the Earth's curvature is considered, using a mathematical representation like a geoid or spheroid.1

Horizontal control often employs triangulation, a system of triangles where angles and at least one side are measured. This creates a network of points. For precise mapping, instruments like the theodolite are used to measure horizontal and vertical angles with high accuracy, establishing a framework of ground marks. Satellite geodesy now refines this knowledge by using satellites alongside land surveys to determine the Earth's precise shape and dimensions.23